With the advancement of information communication technologies, various wireless communication technologies have recently been developed. Among the wireless communication technologies, a wireless local area network (WLAN) is a technology whereby Internet access is possible in a wireless fashion in homes or businesses or in a region providing a specific service by using a portable terminal such as a personal digital assistant (PDA), a laptop computer, a portable multimedia player (PMP), etc.
Ever since the institute of electrical and electronics engineers (IEEE) 802, i.e., a standardization organization for WLAN technologies, was established in February 1980, many standardization works have been conducted.
In the initial WLAN technology, a frequency of 2.4 GHz was used according to the IEEE 802.11 to support a data rate of 1 to 2 Mbps by using frequency hopping, spread spectrum, infrared communication, etc. Recently, the WLAN technology can support a data rate of up to 54 Mbps by using orthogonal frequency division multiplex (OFDM). In addition, the IEEE 802.11 is developing or commercializing standards of various technologies such as quality of service (QoS) improvement, access point (AP) protocol compatibility, security enhancement, radio resource measurement, wireless access in vehicular environments, fast roaming, mesh networks, inter-working with external networks, wireless network management, etc.
In the IEEE 802.11, the IEEE 802.11b supports a data rate of up to 11 Mbps by using a frequency band of 2.4 GHz. The IEEE 802.11a commercialized after the IEEE 802.11b uses a frequency band of 5 GHz instead of the frequency band of 2.4 GHz and thus significantly reduces influence of interference in comparison with the very congested frequency band of 2.4 GHz. In addition, the IEEE 802.11a has improved the data rate to up to 54 Mbps by using the OFDM technology. Disadvantageously, however, the IEEE 802.11a has a shorter communication distance than the IEEE 802.11b. Similarly to the IEEE 802.11b, the IEEE 802.11g implements the data rate of up to 54 Mbps by using the frequency band of 2.4 GHz. Due to its backward compatibility, the IEEE 802.11g is drawing attention, and is advantageous over the IEEE 802.11a in terms of the communication distance.
The IEEE 802.11n is a technical standard relatively recently introduced to overcome a limited data rate which has been considered as a drawback in the WLAN. The IEEE 802.11n is devised to increase network speed and reliability and to extend an operational distance of a wireless network.
More specifically, the IEEE 802.11n supports a high throughput (HT), i.e., a data processing speed of up to 540 Mbps at a frequency band of 5 GHz, and is based on a multiple input and multiple output (MIMO) technique which uses multiple antennas in both a transmitter and a receiver to minimize a transmission error and to optimize a data rate.
In addition, this standard may use a coding scheme which transmits several duplicated copies to increase data reliability and also may use the OFDM to support a higher data rate.
With the widespread use of the WLAN and the diversification of applications using the WLAN, there is a recent demand for a new WLAN system to support a higher throughput than a data processing speed supported by the IEEE 802.11n. A very high throughput (VHT) WLAN system is one of IEEE 802.11 WLAN systems which have recently been proposed to support a data processing speed of 1 Gbps or more. The VHT WLAN system is named arbitrarily. To provide a throughput of 1 Gbps or more, a feasibility test is currently being conducted for the VHT system which uses 4 4 MIMO and a channel bandwidth of 80 MHz or more and which also uses a spatial division multiple access (SDMA) scheme as a channel access scheme.
The conventional channel access mechanism used in the IEEE 802.11n WLAN system or other WLAN systems cannot be directly used as a channel access mechanism of a WLAN system for providing a throughput of 1 Gbps or more (hereinafter, such a WLAN system is referred to as a VHT WLAN system). This is because a channel bandwidth used by the VHT WLAN system is at least 80 MHz since the conventional WLAN system operates under the premise of using a channel bandwidth of 20 MHz or 40 MHz which is too narrow to achieve the throughput of 1 Gbps or more in a service access point (SAP).
Therefore, in order for a VHT basic service set (BSS) to satisfy a total throughput of 1 Gbps or more, several VHT STAs need to simultaneously use a channel in an effective manner. A VHT AP uses SDMA to allow the several VHT STAs to simultaneously use the channel in an effective manner. That is, the several VHT STAs are allowed to simultaneously transmit and receive data to and from the VHT AP.
A modulation and coding scheme (MCS) feedback method is one of methods for more effectively supporting link adaptation in such an IEEE 802.11n MIMO environment. A link adaptation procedure uses a specific MCS with given link quality to increase a data throughput by using a highest transfer rate. However, since the conventional MCS feedback method is achieved under the premise that one-to-one communication is achieved between a station and an AP, the conventional method needs to be compensated when it applies to a multi-user MIMO environment.